i ripped open the sidewall on my front 31" tire. all i have is a spare 30" for now. is it ok to run this or will it shred my gears even if its not in 4 wheel?

I wouldn't run it. Since one OS smaller than the other your diff will soon at different speeds. No good

As long as there is not a locker of some sort in the diff, it will be ok to run the smaller tire for a short time in 2wd. It will cause some weird handling though, so be careful.

the front spider gears will be screaming tho.

Think about it, they give you that tiny doughnut spare from the factory, it's fine to run for a bit till you get a new tire.